Pass string to create_corpus_for_testing

We use the TF test abstractions for the actual corpus tests, which is
where I mostly copied the create_corpus_for_testing calls. Everything
still works, but pytype in some environments will complain since absl
test's create_tempdir() actually returns an internal _Tempdir object
which is not equivalent to a string. Simply access the full_path
property to get a string and make the type checker happy.

compiler_opt/es/blackbox_evaluator_test.py

@@ -56,7 +56,7 @@ def test_trace_get_results(self):
         blackbox_test_utils.ESTraceWorker, count=3, arg='', kwarg='') as pool:
       perturbations = [b'00', b'01', b'10']
       test_corpus = corpus.create_corpus_for_testing(
-          location=self.create_tempdir(),
+          location=self.create_tempdir().full_path,
           elements=[corpus.ModuleSpec(name='name1', size=1)])
       evaluator = blackbox_evaluator.TraceBlackboxEvaluator(
           test_corpus, 5, 'fake_bb_trace_path', 'fake_function_index_path')
@@ -68,7 +68,7 @@ def test_trace_set_baseline(self):
     with local_worker_manager.LocalWorkerPoolManager(
         blackbox_test_utils.ESTraceWorker, count=1, arg='', kwarg='') as pool:
       test_corpus = corpus.create_corpus_for_testing(
-          location=self.create_tempdir(),
+          location=self.create_tempdir().full_path,
           elements=[corpus.ModuleSpec(name='name1', size=1)])
       evaluator = blackbox_evaluator.TraceBlackboxEvaluator(
           test_corpus, 5, 'fake_bb_trace_path', 'fake_function_index_path')